Fiber-optic transmission networks provide transmission for multiple channels using wavelength division multiplexing (WDM). Optical amplifiers, such as erbium-doped fiber amplifiers (EDFAs), provide a mechanism for boosting power after the multiple channels are attenuated due to fiber loss after being transmitted over a distance. Additionally, fiber-optic transmission networks can include other components, such as variable optical attenuators (VOAs), wavelength selective switches (WSSs), wavelength blockers, and the like, which in addition to other functionality can adjust per channel power (or per group of channels).
Referring to FIG. 1, an exemplary fiber-optic network 5 is illustrated showing a single transmission path. Fiber-optic networks typically include both a transmit and receive path for bidirectional communication, but FIG. 1 only depicts a unidirectional path for simpler illustration purposes. Topologies of fiber-optic transmission networks include ring, linear, mesh, and combinations thereof. FIG. 1 illustrates a ring topology with a linear spur through a reconfigurable optical add-drop multiplexer (ROADM) 9. The network 5 is shown with four nodes 400 each including a pre-amplifier 14 located at the input to the nodes and a post-amplifier 18 located at the output of the nodes. The pre-amplifier 14 provides optical amplification prior to de-multiplexing of wavelengths, and the post-amplifier 18 provides optical amplification after multiplexing of wavelengths prior to transmission on a fiber 402.
Each of the nodes 400 illustrated in FIG. 1 include a mid-stage point between the pre- and post-amplifiers 14,18 including a fixed optical add-drop multiplexer (OADM) 6, a variable optical attenuator (VOA) 7, and the ROADM 9. Each of the components 6,7,9 is capable of providing per channel or per group of channels attenuation on wavelengths that are added, dropped, or expressed through the node.
The stability and setting accuracy of optical powers within fiber-optic transmission networks 5 is a universal goal of equipment providers and network operators. The amplifiers 14,18 and attenuation components 6,7,9 each are adjusted to account for different power levels based on a variety of factors, such as channel count. A common obstacle in meeting the stability and setting accuracy goal is the change that occurs in optical gain for channels that are active after a system event. Such an event can include unplanned (e.g., through equipment failure or fiber cut) or planned (e.g., addition or deletion of channels) changes in the number of channels passing through the amplifiers.
Optical power-control-loops are designed to counter individual or collective power changes. For example, these optical power-control-loops are configured to dynamically provide attenuation to individual channels through the various components 6,7,9. However, many concatenated power-control-loops operating in series will result in an over-reaction and an overshoot in the response. Typically, optical power-control-loops are included in fiber-optic transmission networks. These operate at a circuit level in automatic-gain or automatic-power mode to obtain the desired output power into following fibers and devices. With automated power control comes the need for closed control loops. Open-loop, i.e. “set-and-forget”, operation is not possible with the specifications available on many control-point components (e.g., VOAs, wavelength selective switch (WSS) attenuation in ROADMs).
In addition, with the advent of per-channel attenuation provided by wavelength switches and blocker devices, automatic-power control loops are constructed, encompassing multiple optical functions acting in series, to achieve target powers on every wavelength and may counteract wavelength-dependent changes. An example would be the concatenated gain of a per-wavelength attenuation device followed by an optical amplifier device. Such combinations are common in practice.
The shortcoming of a single control loop designed to maintain output power from a node into optical fiber is that when such loops are concatenated in series, control-loops operating in many different nodes react to an event occurring only at one node. The summed reaction is thus greater than needed to compensate for the event and power over-shoot will occur. Alternatively, to minimize overshoot, the control-loop speed may be reduced for all nodes, but the effects of the event are then prolonged.